[lld-macho] Fold __objc_imageinfo sections

Previously, we treated it as a regular ConcatInputSection. However, ld64
actually parses its contents and uses that to synthesize a single image
info struct, generating one 8-byte section instead of `8 * number of
object files with ObjC code`.

I'm not entirely sure what impact this section has on the runtime, so I
just tried to follow ld64's semantics as closely as possible in this
diff. My main motivation though was to reduce binary size.

No significant perf change on chromium_framework on my 16-core Mac Pro:

             base           diff           difference (95% CI)
  sys_time   1.764 ± 0.062  1.748 ± 0.032  [  -2.4% ..   +0.5%]
  user_time  5.112 ± 0.104  5.106 ± 0.046  [  -0.9% ..   +0.7%]
  wall_time  6.111 ± 0.184  6.085 ± 0.076  [  -1.6% ..   +0.8%]
  samples    30             32

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D130125